The present invention relates to an improved method and device for blow molding and shaping hollow articles. In particular, the present invention relates to a highly productive method and device for direct blow molding which allows rapid cooling of articles shaped by parison expansion or blowing.
Blow molding is a technique adopted from the glass industry for molding plastic bottles and other articles from thermoplastic material. The direct blow molding process has been widely used for the shaping of hollow articles made of thermoplastic synthetic resins. It consists of blowing a thin balloon of molten thermoplastic material against the inside walls of a mold and chilling it to a rigid solid. In its most common form, this process includes extruding or injecting a parison downward between the opened halves of a mold, closing the mold to pinch off and seal the parison at top and bottom, injecting air or other fluid through a needle inserted through the parison wall, cooling the mass in contact with the mold, opening the mold, and removing the formed article.
It is difficult to insert a thick needle into the parison to inject air or other fluid because the parison is relatively soft at high temperatures. It is therefore necessary to use a hollow needle having a thin and sharp tip. Air is a fluid and as such is limited in its ability to flow through an orifice. If the air entrance channel is too small, the required blow time is excessively long or the pressure exerted on the parison is inadequate to reproduce the surface details of the mold. Furthermore, while a small amount of air may be adequate for expanding the parison, it might not be enough for cooling the expanded hollow article. Although small articles can be shaped this way, it is not possible to shape large articles.
In the direct blow molding method, a technique used to prevent cosmetic damage to the shaped article includes forming an exhaust opening through an unnecessary section of the article that is cut off after formation to serve as an opening (referred to as the flash section). Another technique includes inserting the hollow needle from an angle perpendicular to the main axis of the parison. In blowing air into the hollow article from this position, it is desirable to blow in the direction of the body of the article, i.e., downward along the axis of the parison, so that the air can flow easily. However, thin needles can only be designed with an opening at the tip of the needle due to the size constraints. A needle bent to a 90 degree angle cannot penetrate the parison. Thus, it is only possible to blow air in a direction perpendicular to the direction of the parison axis, and effective circulation of the air within the parison is prevented. Since a large amount of air for cooling can not be blown into the parison, more time is required for cooling the article.
Cooling is particularly important because it consumes much of the cycle time and therefore bears on product economics. Cooling can take as much as two thirds of the entire "mold-closed" time in a cycle. Results are best when uniform temperatures are maintained throughout the mold. Standard cooling techniques are directed towards either external systems or internal systems or a combination thereof. External systems cool the mold by circulating coolants around the outside or through the walls of the mold. Mold cooling can be improved by increasing the rate of coolant flow through the mold or by making the mold of material with better heat transfer.
Internal systems rely on injecting fluids such as air, a mixture of air and water, or carbon dioxide into the blown part to cool the inside of the parts while they are in the mold. Typical commercial methods include (1) injecting liquid carbon dioxide into the blown part, followed by vaporization, superheating, and exhaustion of the coolant as a gas through the blow pin, (2) injecting highly pressurized moist air into the blown part where it expands to normal blow pressures and produces a cooling effect, (3) passing air through a refrigeration system and into the hot parison, and (4) cycling normal plant air into and out of the blown parts by a series of timers and valves. The size of the hollow needle used for injecting the fluid limits both the speed of the process and the size of articles made using the process.